Method for determining the proportion of a gas constituent contained in a gas mixture

ABSTRACT

An asynchronous motor is used in the process for determining the proportion of a gas constituent, especially water vapor, contained in a gas mixture. The asynchronous motor drives a delivery device for circulating the gas mixture that is used for treating foodstuffs. The difference between the rotational speed, which occurs while the delivery device is driven, and the synchronous rotational speed, i.e. the slip, is used as a measure for the sought proportion of the gas constituent at respectively established pressure and temperature values of the gas mixture.

[0001] The invention relates to a method for determining the proportion of a gas component, in particular steam, in a gas mixture, using a drive-device-driven transport device for the gas mixture.

[0002] To determine gas components in a gas mixture, usually sensors are used. The higher the temperature of the gas, and the more the sensors are exposed to impurities, the more fault-susceptible these sensors become.

[0003] DE 42 06 845 C2 discloses, starting from the known data of a gas mixture, determining the proportion of a gas component, for example steam, by disposing auxiliary blades on a fan for generating pressure differences, in which case, then, from the pressure difference at differing measuring points, the proportion of steam can then be concluded. Since these pressure differences are only very small, pressure difference measuring instruments must be used which have particular sensitivity and accuracy of measurement.

[0004] DE 22 59 459 discloses, in the conditioning of textiles, measuring the steam content of the mixture at regular intervals and correcting it if there is a deviation from the preset value. To measure the mixing ratio of steam and air, a fan is used in a measuring apparatus, the total pressure produced or the power consumption of the fan being determined in this measuring apparatus and this value being used to control the mixing ratio.

[0005] It is an object of the invention, with a cooking apparatus for treating foods, in a simple manner, that is to say using the existing drive apparatus and transport apparatus, to determine the proportion of a gas component, in particular steam, in the gas mixture and thus obtain an index for the moisture.

[0006] This object is achieved by the measures specified in claim 1.

[0007] In this concept of the solution, the fact is exploited that the density of the gas mixture essentially determines the output of the transport device, while the dynamic viscosity of the gas mixture is decisive for the friction losses of the transport device with the gas mixture. If the densities and/or dynamic viscosities of the gas components differ, depending on the composition of the gas mixture, a density and dynamic viscosity which are characteristic for the gas mixture result. In the inventive method, it is then only necessary to record, from appropriate tables available in the literature, the values for density and dynamic viscosity of the two components of a gas mixture in each case at specified pressure and temperature values, as a result for any desired mixing ratio of two components the resultant density and resultant dynamic viscosity of this mixture are obtained. The greater the density and dynamic viscosity of the gas mixture, the more power must a transport device apply under otherwise constant operating parameters.

[0008] A preferred use of the inventive method is that the established parameter is used as control parameter for influencing the composition of the gas mixture, in particular for moisture control of the treatment medium in apparatuses for treating foods. Since in the case of such cooking apparatuses, for example hot air steamers, the temperature in the cooking chamber is determined in any case as an essential state parameter and the atmospheric pressure is ubiquitous pressure information, the invention leads to a particularly simple substantially fault-free economic and in addition still relatively exact method for measuring the composition of the gas mixture of air and steam, and thus the moisture in the cooking apparatus.

[0009] The invention is described in more detail by way of example with reference to the accompanying drawings. In the drawings:

[0010]FIG. 1: shows a section through a cooking apparatus equipped with data analysis for carrying out the method; and

[0011]FIG. 2: shows a section through the line II-II in FIG. 1.

[0012] As can be seen in FIGS. 1 and 2, in a housing 1 of a cooking apparatus, the interior 3 is divided by a partition 2 into a cooking chamber 3 a and an operating chamber 3 b. In the cooking chamber, gratings 4 for holding food to be treated are provided on a rack which is not shown. In the operating chamber 3 b, a fan 5 can be driven by a drive motor 6 that takes in the gas mixture or medium present in the cooking chamber 3 a via a mesh 7 and forces this medium back into the cooking chamber via a heater 8 and slots 9. 10 indicates a steam generator which supplies the steam necessary to generate a steam atmosphere in the cooking chamber 3 a.

[0013] To determine the proportion of dry air or the proportion of steam in the gas mixture, in the embodiment according to FIGS. 1 and 2, a revolution counter 11 and a device 12 for measuring the synchronous speed are provided on the drive motor 6. In addition, a pressure sensor 13 and, within the cooking chamber 3 a, a temperature sensor 14 are disposed. Since the cooking chamber is not pressurized, apart from short-term and negligible pressure differences, the ambient atmospheric pressure always prevails. The atmospheric pressure need not therefore be measured in the cooking chamber, but can be determined somewhere at a particularly suitable position. Generally, a mean atmospheric pressure value corresponding to the elevation of the installation point of the apparatus is also sufficient in determining the composition of the gas mixture, so that in this case the pressure sensor is not necessary. The devices 11 to 14 are connected to a measured data processing and control apparatus 15, termed computer for short hereinafter. This computer can be disposed outside the apparatus 1, as indicated in FIGS. 1 to 3, but is preferably an integrated part of the existing equipment electronics. The usual equipment electronics can generally adopt the necessary functions of the computer 15 without needing to be substantially extended. The computer is programmable and contains the various material data, for example density and dynamic viscosity of dry and moist air at the various temperatures, for example in the temperature range from 20 to 300° C., and the relevant air pressures. In addition, this computer also contains characteristic motor data, so that when a defined slip is determined and when the temperature and pressure are determined the computer is able to determine the corresponding proportions of dry and moist air, which give the moisture in the cooking chamber. In addition, the computer can also contain a program in which defined preset values are stored for moisture at preset temperature values and pressure values. The proportions of the two gas components determined and the moisture thus determined is then compared with the appropriate preset value and, therefrom, via lines 16 and 18 appropriate control commands can then be transmitted to a heater 19 in the steam generator 10 and to a drying apparatus 21, in order to be able to set the composition of the internal atmosphere or cooking chamber atmosphere back to the desired value by supplying steam or removing moisture from the interior 3. The steam is supplied from a steam generator by activating the heater 19 via a slight overpressure of the steam generated through a connection line 20 between steam generator 10 and operating chamber 3 b. The pressure equilibration in the interior is performed exactly as in the case of removal of moisture via connection tubes, which are not shown, between the interior and the surroundings. In the case of removal of moisture, a corresponding amount of gas mixture in the interior is replaced by the ambient air which is always very dry in relation to the cooking chamber atmosphere. 

1. A method for determining the proportion of a gas component, in particular steam, in a gas mixture which is recirculated by means of a transport apparatus for treating foods in a cooking chamber, in which for the drive an asynchronous motor is used, and in which the difference between rotational speed and synchronous speed which is established when the transport apparatus is driven, that is to say the slip, is used as an index of the desired proportion of the gas component at respective determined pressure and temperature values of the gas mixture.
 2. The method as claimed in claim 1, characterized in that the parameter determined is used as control parameter for influencing the composition of the gas mixture, in particular for moisture control of the treatment medium in apparatuses for treating foods. 